Problem: The square root of $x$ is equal to the square of $y$, for $x\geq 0$. If $y = 3$, what is the value of $x$?
Explanation: The first statement in the form of an equation is $\sqrt{x}=y^2$, or $x=y^4$. Using the given value of $y$, we have $x=3^4=\boxed{81}$.